The present invention relates to a process for ion implantation in the production of semiconductor devices etc. and more specifically to an ion implantation process particularly suitable for ion implantation with a high energy (acceleration energy of 500 keV or more).
According to the conventional process of ion implantation in the production of semiconductor devices etc., a photoresist film, a silicon dioxide (hereinafter referred to briefly as "SiO.sub.2 ") film, or the like is used as the mask in ion implantation in desired portions of a semiconductor substrate to selectively implant ions only in the desired portions. However, since the ion energy loss in the mask of a material such as a photoresist or SiO.sub.2 is small, a very thick film having a thickness exceeding a practical limit must be formed in order to exert a sufficient masking effect in ion implantation with a high energy.
This will be explained in further detail. In an example of boron ion (hereinafter referred to briefly as "B.sup.+ ") implantation, the following relationship is generally obtained for resist or SiO.sub.2 materials in providing a complete function of a mask in ion implantation by completely prohibiting penetration of B.sup.+ therethrough: EQU Y=X/85 (1) (1)
wherein the ion implantation energy is X (keV) and the minimum necessary thickness is Y (.mu.m).
For example, a mask having a thickness of 0.6 .mu.m is necessary for B.sup.+ implantation with an acceleration energy of 50 keV. Since the equation (1) is one established with no consideration of variation in practicing the process, a margin of 10% is usually considered necessary in taking into account for both the film thickness and the ion implantation energy to add up to a 20% increase. Thus the actually necessary film thickness of the mask material is 0.72 .mu.m (=0.6.times.1.2).
The film thickness required of a mask in ion implantation with a high acceleration energy is calculated to be found impracticable, for example, about 14 .mu.m (1000/85.times.1.2) with an acceleration energy of 1 MeV. Specifically, it is actually next to impossible to form a film having a thickness of 10 .mu.m or more and a width of 1 .mu.m or less with an accuracy of about 0.1 .mu.m. When the film thickness of the mask is large as mentioned above, there appear portions where no ion is implanted under an influence of the shadow of the mask in the end portions of the mask. This will be explained in a little more detail. As shown in FIG. 2, when an ion beam 1 with an ion beam diameter of a is implanted in a wafer 3 with a radius of r placed at a distance of R from a scanning system 2, a shadow width .DELTA.X at a film thickness d of a mask film 4 in a peripheral portion of the wafer 3 is expressed by the following equation: EQU .DELTA.X =dr/R (2).
On the other hand, a penumbral blur width .DELTA.X' due to an ion beam with a beam diameter of a is expressed by the following equation (3): EQU .DELTA.X'=ar/R (3)
Representative values, i.e. R=1 m, r=6 cm, and a =3 cm, employed in a usual ion implantation apparatus are substituted to calculate .DELTA.X and .DELTA.X', which are found to be 0.7 .mu.m and 0.35 .mu.m, respectively, with a film thickness of 14 .mu.m.
About 1/10 of the minimum feature size is necessary in the dimensional accuracy of an ion-implanted region. For example, when the minimum feature size is 1 .mu.m, the sum of values of .DELTA.X and .DELTA.X' must be about 0.1 .mu.m or less. Accordingly, the film thickness of the mask must be at most 2 .mu.m. However, the conventional mask employed in ion implantation cannot prohibit penetration of ions having a high energy when it has such a small film thickness.